This invention relates to the field of optical communications systems, and more particularly to optical pulse position modulation ("PPM").
In optical communications systems, information must be impressed upon an optical transmission beam. One method of accomplishing this purpose is the controllable modulation (shift) of the relative positions in an input sequence of a stream of optical pulses; that is, different pulses in the sequence are moved forward or backward in time relative to neighboring pulses in accordance with a desired pattern of information, digital or analogue (PPM). Thus, for example, if the input stream is a sequence of equally spaced optical pulses, the output stream will be a sequence of unequally spaced optical pulses, in accordance with a desired digital or an analogue pattern. Thus, pulse position modulators of optical pulses can be used to provide the desired sequence of controllably shifted optical pulses in accordance with a desired pattern of information. A pulse position optical modulator utilizing conventional techniques would consist of an optical delay line, such as a parallel slab of transparent electrooptic material. The refractive index of this electrooptic material can be controllably varied by a voltage applied thereto, so that each of the pulses is controllably delayed upon traversing the electrooptic material in accordance with the instantaneous voltage. However, such a modulator requires an undesirably large amount of electrical power, due to the relatively large voltages required to modulate the refractive index of the electrooptic material, and hence required to modulate the position of the optical pulse traversing therethrough. Moreover, internal pulse position laser modulators, that is, modulators which are located inside the laser resonator, are limited by the rise time of the resonator, typically 10.sup.-.sup.11 sec in semiconductor lasers, whereas the duration of the laser pulse itself is typically only 10.sup.-.sup.12 sec.